REFRACTION OF LIGHT THROUGH A GLASS PRISM
A glass prism is a five-sided solid with a triangular cross-section.
It has two parallel, triangular faces and three rectangular faces that are inclined to each other at some suitable angle, which is called the angle of prism.
In the given fig. ABC is a glass prism of angle A. A ray of light PE is incident on the face AB of the prism an angle ÐPEN1 = i (the angle of incidence). N1N is normal to the face AB at E.
The incident ray PE after refraction at E follows the path EF at an angle ÐFEN = r (the angle of refraction).
Since this ray is going from air (rarer medium) to prism (denser medium), so it bends forwards normal N1N.
The refracted ray EF suffers second refraction at F and follows the path FS at an angle ÐN2FS = e, known as the angle of emergence.
Since it is going from denser medium (glass prism) to rarer medium (air) so it bends away from normal N2N.
The incident ray PE undergoes two refractions, one at point E, while going from air to glass prism and other at point F, while going from glass prism to air.
The path of the ray deviates through an angle ÐHGS = D, known as angle of deviation, on passing through the prism.
The deviation of a ray on passing through a prism depends on angle of prism ‘A’ and angle of incidence ‘i‘ of the ray on one face of the prism.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM
The phenomenon of splitting up of white light into its constituent seven colours on passing through a prism is called dispersion of light.
In the year 1665, Newton discovered that if a beam of white light is passed through a triangular glass prism, the white light splits to form a band of seven colours on a white screen held on the other side of the prism.
This band of seven colours formed on the white screen, when a beam of light is passed through a glass prism, is called spectrum of white light.
The colour sequence obtained on the screen is given by the famous acronym VIBGYOR where :
V stands for Violet
I stands for Indigo
B stands for Blue
G stands for Green
Y stands for Yellow
O stands for Orange
R stands for Red
Violet colour bends through maximum angle whereas the red colour bends the least on passing through the prism, that is why red colour is at the top and violet colour is at the bottom of the spectrum.
RECOMBINATION OF SPECTRUM COLOURS
When a beam of light is passed through a glass prism ABC, it splits up into seven colours.
If this dispersed beam (spectrum) is passed through another identical glass prism say A¢
B¢
C¢ placed alongside the first prism ABC in the opposite direction and in the inverted position on its vertex A¢ then a patch of white light is obtained on the screen placed behind the second prism A‘ B¢
C¢ as shown in the figure.
The recombination of seven colours, produced by the first prism is due to the fact that the refraction produced by second prism A¢
B¢
C¢ is equal and opposite to the refraction produced by the first prism ABC.
FORMATION OF A RAINBOW
A rainbow is a natural spectrum appearing in the sky in the form of an arch of seven colours which is produced by the dispersion of sunlight by tiny water droplets, present in the atmosphere.
A rainbow is always formed in the opposite direction to that of the sun.
The colours come from the dispersion of white light by raindrops suspended in air.
The water droplets act like small prisms. Sunlight entering a drop gets refracted and is split into its component seven colours.
These light rays of the component colours travel through the drop and fall on its other side.
These light rays get reflected and again fall on the surface of the drop and get refracted on the way out. The two refraction bend the light through a large angle, keeping them separate.
These light of different colours emerging from the rain drops forms a rainbow, such that the red colour is at the top and violet colour at the bottom.
Rainbow in the Sky